1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer, and particularly to an image forming apparatus which uses an image transporting belt that holds an image directly or indirectly and moves circularly.
2. Description of the Related Art
Conventionally, as this type of image forming apparatus, there is an intermediate transfer type of image forming apparatus that adopts an electrophotographic system. In this image forming apparatus, a toner image of each color component (for example, Y (yellow), M (magenta), C (cyan), and K (black)) is formed on an image holding member such as a photoconductor drum, and these toner images are temporarily multilayer-transferred on the same part on an intermediate transfer matter (first transfer). Thereafter, this multilayered toner image is transferred on a paper sheet in the lump (second transfer), and next the toner remaining on the intermediate transfer matter is removed by a cleaning device.
As this type of intermediate transfer matter, a belt-like matter is frequently used because it is high in freedom of layout in the apparatus and it is small in occupancy area.
In the mode using this type of intermediate transfer belt, the intermediate transfer belt is laid on a plurality of tension rolls in a tensed state, moves circularly, and transports the multilayer-transferred toner image. However, in case that the velocity of the intermediate transfer belt varies, position errors of the layered toner images of the respective color components are produced, so that a color registration error occurs on the sheet, which causes a technical problem that quality of image lowers.
Due to pressure-contact of a second transfer roll with the intermediate transfer belt in the above image forming cycle, and passing of the sheet between the intermediate transfer belt and the second transfer roll, such the technical problem is caused. Namely, when the second transfer roll is brought into pressure contact with the intermediate transfer belt in order to secondarily transfer the multilayered toner image primarily transferred on the same portion on the intermediate transfer belt, a variation load is applied onto the intermediate transfer belt. Further, a driving load applied on the tension roll to which the drive power is transmitted from the drive source increases, the moving speed of the intermediate transfer belt and the speed of the drive source vary, and elongation of the intermediate transfer belt and strain in the intermediate transfer belt surface are produced by tensile stress acting between the pressure-contact portion and the driving portion.
In order to solve such the technical problems, conventionally a method has been already known (for example, JP-A-11-52757), in which a rotary drive source is provided for a second transfer roll, these are connected through a torque limiter to each other, and the second transfer roll is driven so that its surface speed becomes higher than the surface speed of an intermediate transfer belt, whereby the variation load produced in the intermediate transfer belt is reduced and running performance of the intermediate transfer belt is stabilized.
However, in this method, the variation load in the direction along which the moving speed is accelerated is produced in the intermediate transfer belt, which gives an influence on the moving speed of the intermediate transfer belt.
Further, though the elongation of the intermediate transfer belt by tension power can be prevented, to the contrary, the intermediate transfer belt can change in the contractible direction. particularly, in case where a material that is small in Young's modulus is used in the intermediate transfer belt, the above influence is larger.
Further, in this method, since the torque limiter is necessary for drive torque transmission of the second transfer roll, the structure becomes complicated, and cost becomes high.
The above technical problems are not limited to the intermediate transfer belt but are produced also in belt-like members such as a photoconductor belt and a sheet transporting belt which hold an image directly or indirectly thereon and transport it.